Theoretical investigation of thermodynamic stability and mobility of the oxygen vacancy in ThO2-UO2 solid solutions

The thermodynamic stability and the migration energy barriers of oxygen vacancies in ThO2-UO2 solid solutions are investigated by density functional theory calculations. In pure ThO2, the formation energy of the oxygen vacancy is 7.58 eV and 1.46 eV under O rich and O poor conditions, respectively, while its migration energy barrier is 1.97 eV. The addition of UO2 into ThO2 significantly decreases the energetics of formation and migration of the oxygen vacancy. Among the range of UO2-ThO2 solid solutions studied in this work, UO2 exhibits the lowest formation energy (5.99 eV and -0.13 eV under O rich and O poor conditions, respectively) and Th0.25U0.75O2 exhibits the lowest migration energy barrier (similar to 1 eV). Furthermore, by considering chemical potential, the phase diagram of the oxygen vacancy as a function of both temperature and oxygen partial pressure is shown, which could help to gain experimental control over oxygen vacancy concentration.